[BluePrint Racing]

How bout throwing in why having a lower compression is better when your running forced induction and how that works question. I'll leave the explanation for someone smarter than me.

This is not true... a motor with higher compression will out perform and engine with less compression every time. All the information that has been added to this thread applies to N/A and Forced Induction engines just the same.

A 13:1 engine and 10lbs. of boost will have much greater cylinder pressure then an engine with 8:1 and 10lbs. of boost.

It's an old rule/idea that you cannot mix high compression with boost... this may have been true for my dads generation but not ours. Not with the advanced electronics we have today! Back in the day everyone was limited by fuel as well as ignition control.

 

[Deadphishy]

This is not true... a motor with higher compression will out perform and engine with less compression every time. All the information that has been added to this thread applies to N/A and Forced Induction engines just the same.

A 13:1 engine and 10lbs. of boost will have much greater cylinder pressure then an engine with 8:1 and 10lbs. of boost.

It's an old rule/idea that you cannot mix high compression with boost... this may have been true for my dads generation but not ours. Not with the advanced electronics we have today! Back in the day everyone was limited by fuel as well as ignition control.

True, on the same PSI higher compression will yield more power.

However.

I bet i can pack in more Oxygen doing 13psi on a 10:1 motor, then you can with 10 psi on a 13:1 motor. And still yield the same cylinder pressure while making more power.

 

[BluePrint Racing]

You are comparing apples and oranges now... I'm talking about all things, other then compression, being equal.

The thread is simply talking about compression/ cylinder pressure and it's effect on horsepower, not it's limits for a given enviroment.

 

[Deadphishy]

You are comparing apples and oranges now... I'm talking about all things, other then compression, being equal.

The thread is simply talking about compression/ cylinder pressure and it's effect on horsepower, not it's limits for a given enviroment.

I am just add food for thought for the Statement that Lower compression is better for boost, because you shot it down so quickly. I understand in the context of this thread you are correct.

 

[fourthchirpin]

I dont know if you can use this as an analogy but when you raise the pressure of something that is being combusted the power from that combustion is greater when the pressure is higher and lower when the pressure is lower.


say atmospheric pressure was 30psi on earth instead of 14.7 psi now. if a bomb goes off when the pressure on earth is 30psi that bomb is more powerful then when the ATM pressure is 14.7 psi. I think there is a law that states this more clearly.

 

[Deadphishy]

i think this is the graph that people usally post up when this question is asked.

 

[BSIVOLJR]

Thermal efficiency also depends on what would you consider Thermally efficient? If you consider a naturally aspirated motor only breathing about 1/4th of it's capacity each natural stroke then yes it's efficient In your perspective.
By the way this is true for all of our stock motors...

There are definitely ways to improve...Larger valve's, Porting, Honda engineered Vtec which is amazing in regards to how it increases the Volumetric Efficiency, and also TE. Nothing is better at increasing thermal efficiency in a normal piston engine than Boost(supercharger, Turbocharger), as most know today.

If I had to choose though, definitely a Turbo. Superchargers rob from the crank directly. So they only make about 2/3rds of the potential energy they are capable of. Unless they become independently driven someday? Instantaneous psi on the crank with no spool time 8O...Turbo has proven much more potential boost though. So turbo it is for me. Just studying the different factors that correlate to specific mod's and builds so that I may choose a nice set up.

 

[dcsk8er134]

This is not true... a motor with higher compression will out perform and engine with less compression every time. All the information that has been added to this thread applies to N/A and Forced Induction engines just the same.

A 13:1 engine and 10lbs. of boost will have much greater cylinder pressure then an engine with 8:1 and 10lbs. of boost.

It's an old rule/idea that you cannot mix high compression with boost... this may have been true for my dads generation but not ours. Not with the advanced electronics we have today! Back in the day everyone was limited by fuel as well as ignition control.

i agree, i want to see this tested. if no one else has already done it, i am planning on doing it with my k20 when i go to turbo it, people have boosted stock engines to 600 whp and thats a 11:1 cr. obviously boosting that much on a stock block is possible, but reliability is the only problem. so say someone built the motor so reliability wasnt the issue, then went with a 11.5:1 compression ratio and boosted. lets say we are shooting for a specific power, say 450hp which wouldnt be hard to reach.

so 2 engines, one with a 8:1 cr and one with 11.5:1 make 450 peak hp. the big difference would be in the torque curve. befor boost the engine with 11.5:1 cr will still have good power while the engine with 8:1 cr would have alot less power. also you could get rid of alot of lag in the 11.5:1 cr engine because less boost will be needed to reach 450hp, so u could use a smaller faster spooling turbo. and even with the same size turbo peak boost would come earlyer on the higher compression engine.

so in the end a higher compression ratio is better for n/a or forced induction.

 

[Deadphishy]

so in the end a higher compression ratio is better for n/a or forced induction.

If you have a set, low Hp goal then yes.

The only major difference will be that the high compression motor will have much Higher inner cylinder pressures. Which will put more stress on the motor.

I would still rather use a few extra PSI to make the same power with less overall pressure.

It really depends on what you are doing with the motor, your goals and needs.


Doesn't really need to be tested. A lot of people and race teams run high compression boosted motors. They make plenty of power.

 

[BSIVOLJR]

i agree, i want to see this tested. if no one else has already done it, i am planning on doing it with my k20 when i go to turbo it, people have boosted stock engines to 600 whp and thats a 11:1 cr. obviously boosting that much on a stock block is possible, but reliability is the only problem. so say someone built the motor so reliability wasnt the issue, then went with a 11.5:1 compression ratio and boosted. lets say we are shooting for a specific power, say 450hp which wouldnt be hard to reach.

so 2 engines, one with a 8:1 cr and one with 11.5:1 make 450 peak hp. the big difference would be in the torque curve. befor boost the engine with 11.5:1 cr will still have good power while the engine with 8:1 cr would have alot less power. also you could get rid of alot of lag in the 11.5:1 cr engine because less boost will be needed to reach 450hp, so u could use a smaller faster spooling turbo. and even with the same size turbo peak boost would come earlyer on the higher compression engine.

so in the end a higher compression ratio is better for n/a or forced induction.

The idea is right but a little misleading IMO... if the compression ratio is roughly a little higher in the NA model the PSI could be close but the cylinder pressure in the expansion cylinder will be much higher in the TC motor. An NA motor's PSI will increase gradually and consistently...A boosted motor will exponentially increase unlike a NA motor resulting in much higher cylinder pressure throughout the powerband. More Cylinder pressure=More torque. Boosted 300hp and NA 300hp are two very different things. Although boosting can cause failure in lacking components much quicker due to exponential power. A high power NA motor will break healthy good working components much faster than a turbocharged motor.

 

[lzyga05]

If you have a set, low Hp goal then yes.

The only major difference will be that the high compression motor will have much Higher inner cylinder pressures. Which will put more stress on the motor.

I would still rather use a few extra PSI to make the same power with less overall pressure. .

I don't understand this, i thought if you had boost on say 8:1 pistons your cylinder pressures would be the same if you had like a 12:1 pistons with no boost

 

[Deadphishy]

I don't understand this, i thought if you had boost on say 8:1 pistons your cylinder pressures would be the same if you had like a 12:1 pistons with no boost

Overall pressure might be close with low boost. However. Lower compression allows for a larger combustion volume which means more air, which means more power. But that only matters if air is being Forced into the motor, to fill the otherwise dead air space.

8:1 with 10 PSI before compression = 80psi after or 197.6 PSI absolute pressure
12:1 with 10 PSI before compression = 120psi after or 296.4 PSI absolute pressure
12:1 with STP before compression( 14.7psi) = 176.4PSI absolute pressure

If my fuzzy math is correct. Someone please correct this if i'm missing something

 

[Deadphishy]

I don't understand this, i thought if you had boost on say 8:1 pistons your cylinder pressures would be the same if you had like a 12:1 pistons with no boost

Sorry i was talking if both motors were boosted. but still making the same HP

 

[BSIVOLJR]

I don't understand this, i thought if you had boost on say 8:1 pistons your cylinder pressures would be the same if you had like a 12:1 pistons with no boost

(Hypothetically speaking) your right. Your idea is right. As I stated in my prior post I thought that post was a little misleading. I think better said...2 motors of equal power one NA one TC will not differ in peak cylinder pressure much at all but the TC motor will have a much higher CP throughout the stroke...Remember air is constantly being forced in with forced induction so the pressure will be more throughout the stroke than a Naturally induced Cylinder...Again this increased cylinder pressure through out the stroke creates much more torque and similar hp obviously these are motors built for similar power as mentioned originally. Again this is (hypothetically speaking)

Overall pressure might be close with low boost. However. Lower compression allows for a larger combustion volume which means more air, which means more power. But that only matters if air is being Forced into the motor, to fill the otherwise dead air space.

8:1 with 10 PSI before compression = 80psi after or 197.6 PSI absolute pressure
12:1 with 10 PSI before compression = 120psi after or 296.4 PSI absolute pressure
12:1 with STP before compression( 14.7psi) = 176.4PSI absolute pressure

If my fuzzy math is correct. Someone please correct this if i'm missing something

EXACTLY

 

[radek]

This is not true... a motor with higher compression will out perform and engine with less compression every time. All the information that has been added to this thread applies to N/A and Forced Induction engines just the same.

A 13:1 engine and 10lbs. of boost will have much greater cylinder pressure then an engine with 8:1 and 10lbs. of boost.

It's an old rule/idea that you cannot mix high compression with boost... this may have been true for my dads generation but not ours. Not with the advanced electronics we have today! Back in the day everyone was limited by fuel as well as ignition control.

The reason why people lower the compression ratio on a forced induction engine is because of knocking... When the engineers are calibrating the engine, they calibrate something called SBP, that stands for Spark Advance for Best Torque, so the engine is on its knocking limit, and if you install a turbocharger in a factory engine you almost certainly get heavy knocking unless you decrease the compression ratio.

Theorethicaly a higher compression ratio will get you better thermal efficiency, but it is not always the right way to go. Sometimes when you increase the compression ratio, lets say of a NA engine, you can get an engine that does not respond well to spark advance, and that is shit if you want to extract the best performance of the engine.

Also, to explain why the engines performance will improve with higher compression ratio, whe can look at thermodynamics principles. The higher the compression ratio, the hotter the air is at the end of the compression cycle, and that will help you in a number of ways, one of them is the higher vaporitzation of the fuel, and that is one of the main issues on ICE, for combustion will be effective only on gas mixtures, not on liquid gas mixtures, and even with the modern day technology we still dont get 100% fuel vaporization. Another point is, if you have studied thermodynamics you learned that for example in a Rankine cycle thermal efficiency is higher when you preheat the air from combustion, the same applies to ICE. You could ask then why the **** do we use intercoolers? The answer to that has nothing to do with we are talking about, for we are talking about temperatures after compression and on the intercooler case, intake air temperature, which are two different topics

 

[lzyga05]

thats why we buy hondata and tune with that

 

[mrblah]

How bout throwing in why having a lower compression is better when your running forced induction and how that works question. I'll leave the explanation for someone smarter than me.

I believe you are referring to the compression the manufacturer is giving you (like putting 9:1 comp pistons to go FI). By doing so, you get a bigger combustion chamber (or so my understanding lead's me to believe so.).

But that doesn't mean you have less internal pressure in the combustion chamber!!! remember you are pressurizing the air before it enters the chamber. Basically you increase volume AND pressure, and if done correcly you get more air and fuel in there.

Since the only compression happening in an NA engine is done by the piston, you want it to compress as much as you can to cen better burning, and more fuel (more pressure = more air so you can dump more fuel). this is why NA typically needs higher static CR.



Let's not confuse the static CR, dynamic CR and actual internal pressure.

 

[sethgreen]

I know I'm new and this is a KA forum but im having some questions of my own after reading this thread.

Im running a F22a4 block with a F22b1 head
CR is 11:1
Bisimoto nitrous duty parts including valves, springs, retainers, camshaft etc etc.
Head has been ported
P28 ecu tuned on Crome

Lets say I didn't want to throw nitrous in it and wanted to go turbo.
Most people tell me that, its impossible b.c of my compression. They say I would have to run race gas only, or get a completely spot on tune.
But after reading, I'm not sure if that's right or not. Input?

 

[BSIVOLJR]

I know I'm new and this is a KA forum but im having some questions of my own after reading this thread.

Im running a F22a4 block with a F22b1 head
CR is 11:1
Bisimoto nitrous duty parts including valves, springs, retainers, camshaft etc etc.
Head has been ported
P28 ecu tuned on Crome

Lets say I didn't want to throw nitrous in it and wanted to go turbo.
Most people tell me that, its impossible b.c of my compression. They say I would have to run race gas only, or get a completely spot on tune.
But after reading, I'm not sure if that's right or not. Input?

You could boost on 93 or 91 but you'd need a good tuner cause the tq curve would be crazy

 

[sethgreen]

Well the guy who did the tune, tuned it for all motor. Hes regarded as one of the best tuners is missouri. But im sure about the quality of Crome.